Calbindin D28k targets myo -inositol monophosphatase in spines and dendrites of cerebellar Purkinje neurons

Abstract

The Ca 2؉ -binding protein calbindin D28k (CB) is vital for the normal function of the central nervous system but its specific functional role is largely unclear. CB is typically described as a mobile Ca 2؉ buffer that shapes the spatiotemporal extent of cellular Ca 2؉ signals. Recent biochemical data, however, indicate that CB also has characteristics of a Ca 2؉ sensor and activates myo-inositol monophosphatase (IMPase), a key enzyme of the inositol-1,4,5-trisphosphate signaling cascade and an assumed target of moodstabilizing drugs in the treatment of bipolar disorder. Here, we show that CB interacts with IMPase in cerebellar Purkinje neurons, a cell type well known to rely on inositol-1,4,5-trisphosphatedependent synaptic integration. Quantification of the mobility of dye-labeled CB with two-photon fluorescence recovery after photobleaching revealed that a substantial fraction of CB is immobilized in spines and dendrites, but not in axons. Immobilization occurs over several seconds, is increased by suprathreshold synaptic activity, and can be relieved by a synthetic peptide that resembles the putative CB-binding site of IMPase, indicating that CB binds to immobilized IMPase. Measurements of the apparent diffusion coefficients of CB imply that CB does not interact with cytosolic IMPase or that the latter is present only in minute amounts in the spiny dendrites of Purkinje neurons. Our results suggest that CB acts as an activity-dependent sensor that targets membrane͞cytoskeleton-bound IMPase in central neurons.calcium ͉ inositol-1,4,5-trisphosphate ͉ two-photon microscopy ͉ diffusion ͉ mobility

Cytosolic Ca2+ Buffers

“…The latter value is very similar to PV's mobility in frog myoplasm, 43 and 32 mm 2 /s for transverse and longitudinal diffusion, respectively (Maughan and Godt 1999), but clearly smaller than in an aqueous solution: 140 mm 2 /s (Feher 1984). As expected from the larger M r of CB-D28k (%29 kDa) compared to PV (%12 kDa), D in Purkinje cell dendrites is smaller: 26 mm 2 /s (Schmidt et al 2005). How the presence of a Ca 2þ buffer affects the spatiotemporal aspects of a cytosolic Ca 2þ transient not only depends on the mobility of the Ca 2þ buffer, but also on the mobility of the free Ca 2þ ions.…”

“…In most cases, binding studies were performed in vitro; the physiological implications of these interactions are not clear yet. In dendrites and spines of Purkinje cells, approximately 20% of CB-D28k molecules are temporarily, that is, for several seconds, immobile by their binding to IMPase, a key enzyme of the InsP 3 -signaling cascade, and the fraction of immobilized CB-D28k increases by climbing fiber stimulation (Schmidt et al 2005). In summary, the above findings, together with CB-D28k's Ca 2þ -dependent conformational changes, indicate additional Ca 2þ sensor functions (Berggard et al 2002a).…”

“…o CB-D28k's mobility in PC dendrites is 26 mm 2 s 21 (Schmidt et al 2005), clearly slower than in water (Gabso et al 1997), and also slower than PV in PC dendrites. buffers act in different ways to modulate the spatiotemporal aspects of cytosolic Ca 2þ signals.…”

Differences in locomotor behavior revealed in mice deficient for the calcium-binding proteins parvalbumin, calbindin D-28k or both

“…They are implicated to contribute to the regulation of Ca 2+ homeostasis by acting as cytosolic Ca 2+ buffers [4][5][6] and recent experiments on CB suggest that this protein might have additional functions as a Ca 2+ sensor [7,8]. PV is almost exclusively expressed in inhibitory GABAergic interneurons in different regions including the neocortex, cerebellum, hippocampus and the reticular nucleus of the thalamus (RTN) of the mammalian brain and also in few projection neurons.…”

A Sensitive Period of Mice Inhibitory System to Neonatal GABA Enhancement by Vigabatrin is Brain Region Dependent

“…The increase in CB levels in the hippocampi of GVG-treated newborn and adult mice, without any corresponding alteration in the density or size of CB-IR neurons, might indicate an increase in cellular levels of this protein. The CB protein serves as an endogenous fast Ca 2 + buffer (Nägerl et al, 2000) and thus it regulates activity-dependent processes in synapses (Blatow et al, 2003;Schmidt et al, 2005) and protein activation (Berggard et al, 2002). Therefore, modified cellular CB levels in GVG-treated mice may result in a change in synaptic strength, excitability, and modulation of signaling pathways involving Ca 2 + .…”